1. Field of the Invention
This invention relates to a planar monolithic filter array suitable for use in an electrical connector, and to an electrical connector utilizing such an array.
2. Description of Related Art
It is known to provide electrical filters in electrical connectors for the purpose of protecting sensitive electrical components from transient currents and voltages which develop in a transmission cable due to electromagnetic and radio frequency interference. Such transients are generally high frequency waveforms, and therefore capacitive or tuned pi circuits may be used to shunt the transients to ground without affecting the primary signal carried by the cable.
Conventional electrical connector capacitive filter designs generally fall into one of two categories: monolithic planar capacitors, and tubular capacitor arrangements. Monolithic planar capacitors are more convenient to install in an electrical connector, but lack the flexibility of tubular capacitor arrangements because it is difficult to vary capacitances between pins, while electrical connectors utilizing tubular capacitor arrangements tend to be more costly and difficult to assemble than those utilizing conventional monolithic planar capacitors even thought they offer the advantage of customized capacitances for different pins.
As illustrated in FIGS. 1 and 2, monolithic planar capacitors consist of a generally planar block 201 of dielectric material that includes interleaved ground electrodes 202 and live electrodes 203. Live electrodes 203 are arranged to be electrically connected to contact pins 204 (only one of which is shown) inserted through holes 205 in the block of dielectric material. The dielectric material is typically a ceramic material such as barium titanate. Holes 205 are conductively plated or include some type of conductive sleeve to form terminals 206, and either the contact pins 204 or the terminals 206 may be arranged to include or form a resilient structure 207 for maintaining an electrical connection between the contact pins and the terminals when one of the contact pins is inserted in the corresponding one of holes 205. The ground electrodes 202 extend to the perimeter of the array and are joined to a termination strip or coating 208 which may be electrically connected to ground via the shell 209 of a connector and/or via connecting structures such as plate springs 210.
As noted above, a problem with the monolithic capacitor structure of the prior art is that the range of capacitances that can be provided is relatively limited. The capacitance values achieved by this type of planar array capacitor are determined by the area of overlap of the active and ground electrodes, the thickness of the dielectric material between the electrodes and the dielectric constant of the material, but in practice the main limiting factor in determining the range of capacitances possible within the array is the dielectric constant of the ceramic or other dielectric material. By simply changing the overlap area of the electrodes or the dielectric thickness, a ratio of maximum to minimum capacitances of about 10:1 is all that can be achieved. While it is possible to form the dielectric material by co-firing different ceramic materials within one array to extend the range of capacitances, the associated processing problems make this an impractical solution.
In addition to the problem of limited range, the monolithic capacitor structure has the disadvantages that (i) the entire filter unit must be replaced as a unit even if only one of the capacitors is defective or needs to changed, (ii) in order to form pi filters, the inductors must be in tubular form and surrounded by the capacitors, which can create problems because of the different thermal expansion and other characteristics of the ceramic dielectric material of the filter block and the typically ferrite material of the inductors, and (iii) it is virtually impossible to eliminate cross-talk between the contacts through the common capacitances.
The second category of prior art capacitive filter designs involves utilizing discrete cylindrical or tubular capacitors which are formed as filter sleeves to be fitted around the contact pins and grounded to the connector shell through a conductive metal or metallized ground plate. These designs are disadvantageous in that the filter sleeves are fragile and require relatively complicated grounding and support arrangements, the use of capacitive filter sleeves leaves less room for the addition of inductive filter sleeves, although relative complicated combined inductive and capacitive filter sleeves are known.
In order to eliminate the disadvantages of conventional monolithic block capacitors and discrete cylindrical capacitive filter designs, it was proposed in U.S. Pat. No. 5,153,540 to provide an alternative monolithic filter capacitor design in which the dielectric substrate and interleaved electrodes are replaced by a monolithic supporting structure arranged to accommodate individual discoidal capacitors for each contact to be filtered. The supporting structure is in the form of a single high strength metal oxide substrate having a plurality of counter bores on each side and a connecting bore between the respective counter bores for accommodating low cost discoidal capacitors and ferrite inductors, as necessary, thereby providing a more robust monolithic capacitor array that has the advantage of being as easily assembled to a filter capacitor as the prior monolithic capacitor arrays, while permitting both capacitance values and filter types to be easily mixed within the array.
While the capacitor array disclosed in U.S. Pat. No. 5,153,540 is advantageous for many applications because it combines the strength and case of assembly of a monolithic capacitor array with the versatility of individual discoidal capacitors for each contact pin, a number of further improvements could still be made. First, in the capacitor array of U.S. Pat. No. 5,153,540, each contact to be filtered must be provided with an individual discoidal capacitor even though most contacts in a typical connector can use identical capacitances. Second, the relatively small discoidal capacitors required in a high density connector also have a limited range of capacitances, albeit greater than the range possible with the conventional monolithic capacitor designs. Third, bonding of the discoidal capacitors to the metal oxide support plate can create stresses in the capacitors due to the different thermal characteristics of the support structure and the discrete capacitors.
Therefore, a need still exists for an electrical connector filter assembly that provides a range of capacitances greater than is possible with either the conventional monolithic or discrete capacitive filter designs, and yet combines the ease-of-assembly of a conventional monolithic filter with the versatility of a discrete capacitors, with the added advantage of providing discrete capacitors or filters for only those of the contacts requiring filtering, and of enabling the materials of the support structure to be more closely matched to those of the discrete filter elements.
The claimed invention provides such a filter assembly by arranging the conventional capacitive filter structure to accommodate discrete capacitors. Those skilled in the art will recognize that it has previously been proposed to combine monolithic capacitor structures with filter elements such as inductors and/or diodes, as disclosed for example in U.S. Pat. Nos. 4,083,022 and 4,707,048, but none of the previous proposals have included the suggestion that the conventional monolithic capacitor structures be combined with additional discrete capacitors or capacitive filters rather than inductive or semiconductor type components to achieve the advantages noted above.